Glycoproteins such as the fibrillar collagens, types I-III, are some of the main proteins in the extracellular matrix (ECM). Collagen is attached to specific cell-surface receptors that have the amino acid sequence Arg-Gly-Asp (Albelda & Buck, 1990; Dedhar, et al., 1987; and Ruoslahti & Engvall, 1994); specifically this sequence known as the “RGD motif” has been implicated as the cell attachment site of such ECM proteins as fibronectin, vitronectin, fibrinogen, and von Willebrand factor, and is also present in type I collagen.
Because collagen is a biocompatible glycoprotein, there has been interest in developing collagenous drug carriers that can be loaded with drugs and other bioactive agents.
There are two basic classes of drug carriers (Bangham, 1993; Benita & Levy, 1993; Gref et al., 1994; and Wu et al., 1994): (1) particulate systems, such as cells, microspheres, viral envelopes, and liposomes and (2) non-particulate, usually soluble, systems consisting of macromolecules such as proteins or synthetic polymers.
Microscopic and macroscopic particulate carriers have several distinct advantages over treatment with free drugs and non-particulate carriers. They can perform as sustained-release or controlled-release drug depots, thus contributing to improvement in drug efficacy and allowing reduction in the frequency of dosing. By protecting both the entrapped-drug and the biological environment, these carriers reduce the risks of drug inactivation and degradation. Since the pharmacokinetics of free drug release from the depots are different than from directly-administered free drug, these carriers have the potential to reduce toxicity and undesirable side effects.
Despite the advantages offered, the use of currently existing drug encapsulating particulate carriers has posed some challenges which have yet to be fully overcome. For example, both macroparticulate and microparticulate drug delivery systems display limited targeting abilities; limited retention and stability in circulation; potential toxicity upon chronic administration; and an inability to extravasate. Numerous attempts have been made to bind substances such as antibodies, glycoproteins and lectins to particulate systems (e.g., liposomes, microspheres and others) in order to improve targeting ability.
Although bonding of these targeting agents to the particulate systems has met with success, the resulting modified particulate systems have not performed as hoped, particularly in vivo. Other difficulties are also present. For example, for maximal effectiveness, antibodies should be patient-specific and therefore add cost to the therapeutic regimen.
Further, not all binding between the targeting substance and the carrier is covalent. This type of bond is essential, as non-covalent binding might result in dissociation of the targeting substances from the particulate system at the site of administration, due to competition between the particulate system and the targeted components at the site. Upon such dissociation, the administered modified particulate system would likely revert to a conventional particulate system, thereby defeating the purpose of administration of the modified particulate system.
Therefore, there is a need in the art for a novel adhesive biopolymer that can serve as a particulate carrier of drugs and other bioactive agents. Such a biopolymer would be fully degradable and compatible in and with biological systems, unlike existing particulate carriers that have non-biological components. Due to the use of biocompatible raw materials, this biopolymer would be nontoxic and nonimmunogenic, unlike some of the existing carriers. (Toxicity and immunogenicity varies from one carrier to another, but is on an acceptable level in those few systems approved for clinical use.).
The novel bioadhesive polymer serving as a particulate carrier should also demonstrate high-efficiency entrapment independent of drug size up to and including proteins and genetic material, due to a “wraparound” or “induced-fit” nature. Existing particulate carriers demonstrate variable entrapment efficiencies ranging from low to high, with low efficiency of high molecular weight entities. For intended use as a depot, it would be desirable if the novel biopolymer exhibited exceptionally slow drug efflux, with a half-life in the range of 2-15 days. This would be in contrast to the high variability seen in conventional preparations, with drug efflux ranging from fast to slow.
Additionally, it would be advantageous if the bioadhesive nature of the glycoprotein component would endow the system with the ability to adhere with high affinity to in vivo recognition sites and confer a measure of active targeting, in contrast with the conventional preparations. In conventional preparations, further carrier modification is required to endow the systems with these properties, but is not always feasible and in some cases is counter-productive to production and to the intended in vivo fate. It would be further desirable if the biopolymer comprised a glycoprotein, such as collagen, to which a lipid such as phosphatidylethanolamine were covalently linked.